Mercury (Hg), a well-known toxin, is essentially nontoxic in its elemental form. However, once it is chemically modified to the ionized, inorganic species, Hg(++), it becomes toxic. Further bioconversion to an alkyl Hg, such as methyl Hg (CH[3]Hg[+]), yields a species of Hg that is highly selective for lipid-rich tissue, such as the myelin sheath, and is very toxic.